adding Explanation on how to pass in metadata in the docs

[Nelwhix]

Thanks for your awesome package! I wanted to pass in some metadata with a successful transaction earlier today. Then I noticed there was no explanation on the docs. I was about to write a custom wrapper till I read through the code and found that your wrapper actually takes in those paramaters.

This fix to the docs is for future users who may want to know how they could quickly set it up